Charge distribution guided by grain crystallographic orientations in polycrystalline battery materials
Zhengrui Xu, Zhisen Jiang, Chunguang Kuai, Rong Xu, Changdong Qin, Yan Zhang, Muhammad Mominur Rahman, Chenxi Wei, Dennis Nordlund, Cheng-Jun Sun, Xianghui Xiao, Xi-Wen Du, Kejie Zhao, Pengfei Yan, Yijin Liu & Feng Lin

Nature Communications volume 11, Article number: 83 (2020) Cite this article

Abstract
Architecting grain crystallographic orientation can modulate charge distribution and chemomechanical properties for enhancing the performance of polycrystalline battery materials. However, probing the interplay between charge distribution, grain crystallographic orientation, and performance remains a daunting challenge. Herein, we elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic “surface-to-bulk” charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.

Introduction
Solid state redox reactions are ubiquitous during ion reactions in batteries1 and catalysts2. Understanding how redox reactions propagate can inform designing internal microstructures such that redox reactions can be fully accessed to deliver the desired functionalities. The propagation of redox reactions governs the electrochemical properties of battery materials and their critical performance metrics in practical cells3,4,5. The recent research progress, especially aided by advanced analytical techniques6, has revealed that incomplete and heterogeneous redox reactions prevail in many electrode materials, such as olivine phosphates7,8,9,10,11,12,13,14, layered oxides15,16,17,18,19,20, spinel oxides21,22, and conversion materials23,24. Advanced high-capacity cathode materials for lithium (Li) ion and sodium (Na) ion batteries are mostly polycrystalline materials that exhibit complex charge distribution (the valence state distribution of the redox-active cations) due to the presence of numerous constituting grains and grain boundaries. The redox reactions in individual grains typically do not proceed concurrently due to their distinct geometric locations in polycrystalline particles. As a result, these unsynchronized local redox events collectively induce heterogeneous and anisotropic charge distribution, building up intergranular and intragranular stress25. Therefore, these polycrystalline materials may exhibit weak mechanical stability, leading to undesired chemomechanical breakdown during battery cycling26,27,28,29.
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